The present invention is directed to a method and apparatus which is used in conjunction with conventional downhole measurement-while-drilling equipment to establish communication therewith while the equipment sub is on the surface.
Recent developments in the area of measurement-while-drilling equipment has produced sufficient technological changes in that the downhole equipment is able to accumulate a substantial amount of information, such as detailed descriptions of the various strata through which the drill is passing. Much of this information is extremely important, but not essential to the on-going drilling operation. The drilling operation would require all of the details of the drill bit orientation, so as to correct the advancing boring operation and would be less concerned with the actual strata being penetrated. Thus, the usual telemetry that is provided, either with a mud pulsing system or hard wired system would give priority to the boring information and store the more detailed information concerning the various strata. Other known downhole equipment only records formation data for subsequent recovery and evaluation. It is then possible to recover any stored information when the downhole equipment is periodically returned to the surface for bit replacement.
Prior systems for recovery of the stored data have employed direct electrical connections, which require an aperture to be formed in the wall of the downhole equipment sub. Having an aperture in the wall of the sub creates the problem of weakening the structure creating the possibility of fracturing the sub with the possible resulting loss of the downhole equipment requiring an expensive fishing recovery operation. Other prior art systems have required breaking open of the drill string, at the surface, to either recover recording means itself or gain access to the recording made while downhole.
Measurement-while-drilling resistivity sensors are generally transmitters and receivers of either the current (conduction) type or induction type. Current type sensors include electrodes which are mounted on the surface of the sub for injecting and receiving current directly into and from the drilling mud and formation immediately adjacent to the sub. Induction type sensors include antenna coils on the surface of the sub for inducing and detecting currents into and from the surrounding drilling fluid and formation.